diff --git a/src/applications/maniphest/config/PhabricatorManiphestConfigOptions.php b/src/applications/maniphest/config/PhabricatorManiphestConfigOptions.php
index 077fc511ce..60a41a26ca 100644
--- a/src/applications/maniphest/config/PhabricatorManiphestConfigOptions.php
+++ b/src/applications/maniphest/config/PhabricatorManiphestConfigOptions.php
@@ -1,503 +1,530 @@
 <?php
 
 final class PhabricatorManiphestConfigOptions
   extends PhabricatorApplicationConfigOptions {
 
   public function getName() {
     return pht('Maniphest');
   }
 
   public function getDescription() {
     return pht('Configure Maniphest.');
   }
 
   public function getIcon() {
     return 'fa-anchor';
   }
 
   public function getGroup() {
     return 'apps';
   }
 
   public function getOptions() {
     $priority_type = 'maniphest.priorities';
     $priority_defaults = array(
       100 => array(
         'name'  => pht('Unbreak Now!'),
         'keywords' => array('unbreak'),
         'short' => pht('Unbreak!'),
         'color' => 'pink',
       ),
       90 => array(
         'name' => pht('Needs Triage'),
         'keywords' => array('triage'),
         'short' => pht('Triage'),
         'color' => 'violet',
       ),
       80 => array(
         'name' => pht('High'),
         'keywords' => array('high'),
         'short' => pht('High'),
         'color' => 'red',
       ),
       50 => array(
         'name' => pht('Normal'),
         'keywords' => array('normal'),
         'short' => pht('Normal'),
         'color' => 'orange',
       ),
       25 => array(
         'name' => pht('Low'),
         'keywords' => array('low'),
         'short' => pht('Low'),
         'color' => 'yellow',
       ),
       0 => array(
         'name' => pht('Wishlist'),
         'keywords' => array('wish', 'wishlist'),
         'short' => pht('Wish'),
         'color' => 'sky',
       ),
     );
 
     $status_type = 'maniphest.statuses';
     $status_defaults = array(
       'open' => array(
         'name' => pht('Open'),
         'special' => ManiphestTaskStatus::SPECIAL_DEFAULT,
         'prefixes' => array(
           'open',
           'opens',
           'reopen',
           'reopens',
         ),
       ),
       'resolved' => array(
         'name' => pht('Resolved'),
         'name.full' => pht('Closed, Resolved'),
         'closed' => true,
         'special' => ManiphestTaskStatus::SPECIAL_CLOSED,
         'transaction.icon' => 'fa-check-circle',
         'prefixes' => array(
           'closed',
           'closes',
           'close',
           'fix',
           'fixes',
           'fixed',
           'resolve',
           'resolves',
           'resolved',
         ),
         'suffixes' => array(
           'as resolved',
           'as fixed',
         ),
         'keywords' => array('closed', 'fixed', 'resolved'),
       ),
       'wontfix' => array(
         'name' => pht('Wontfix'),
         'name.full' => pht('Closed, Wontfix'),
         'transaction.icon' => 'fa-ban',
         'closed' => true,
         'prefixes' => array(
           'wontfix',
           'wontfixes',
           'wontfixed',
         ),
         'suffixes' => array(
           'as wontfix',
         ),
       ),
       'invalid' => array(
         'name' => pht('Invalid'),
         'name.full' => pht('Closed, Invalid'),
         'transaction.icon' => 'fa-minus-circle',
         'closed' => true,
         'claim' => false,
         'prefixes' => array(
           'invalidate',
           'invalidates',
           'invalidated',
         ),
         'suffixes' => array(
           'as invalid',
         ),
       ),
       'duplicate' => array(
         'name' => pht('Duplicate'),
         'name.full' => pht('Closed, Duplicate'),
         'transaction.icon' => 'fa-files-o',
         'special' => ManiphestTaskStatus::SPECIAL_DUPLICATE,
         'closed' => true,
         'claim' => false,
       ),
       'spite' => array(
         'name' => pht('Spite'),
         'name.full' => pht('Closed, Spite'),
         'name.action' => pht('Spited'),
         'transaction.icon' => 'fa-thumbs-o-down',
         'silly' => true,
         'closed' => true,
         'prefixes' => array(
           'spite',
           'spites',
           'spited',
         ),
         'suffixes' => array(
           'out of spite',
           'as spite',
         ),
       ),
     );
 
     $status_description = $this->deformat(pht(<<<EOTEXT
 Allows you to edit, add, or remove the task statuses available in Maniphest,
 like "Open", "Resolved" and "Invalid". The configuration should contain a map
 of status constants to status specifications (see defaults below for examples).
 
 The constant for each status should be 1-12 characters long and  contain only
 lowercase letters and digits. Valid examples are "open", "closed", and
 "invalid". Users will not normally see these values.
 
 The keys you can provide in a specification are:
 
   - `name` //Required string.// Name of the status, like "Invalid".
   - `name.full` //Optional string.// Longer name, like "Closed, Invalid". This
     appears on the task detail view in the header.
   - `name.action` //Optional string.// Action name for email subjects, like
     "Marked Invalid".
   - `closed` //Optional bool.// Statuses are either "open" or "closed".
     Specifying `true` here will mark the status as closed (like "Resolved" or
     "Invalid"). By default, statuses are open.
   - `special` //Optional string.// Mark this status as special. The special
     statuses are:
     - `default` This is the default status for newly created tasks. You must
       designate one status as default, and it must be an open status.
     - `closed` This is the default status for closed tasks (for example, tasks
       closed via the "!close" action in email or via the quick close button in
       Maniphest). You must designate one status as the default closed status,
       and it must be a closed status.
     - `duplicate` This is the status used when tasks are merged into one
       another as duplicates. You must designate one status for duplicates,
       and it must be a closed status.
   - `transaction.icon` //Optional string.// Allows you to choose a different
     icon to use for this status when showing status changes in the transaction
     log. Please see UIExamples, Icons and Images for a list.
   - `transaction.color` //Optional string.// Allows you to choose a different
     color to use for this status when showing status changes in the transaction
     log.
   - `silly` //Optional bool.// Marks this status as silly, and thus wholly
     inappropriate for use by serious businesses.
   - `prefixes` //Optional list<string>.// Allows you to specify a list of
     text prefixes which will trigger a task transition into this status
     when mentioned in a commit message. For example, providing "closes" here
     will allow users to move tasks to this status by writing `Closes T123` in
     commit messages.
   - `suffixes` //Optional list<string>.// Allows you to specify a list of
     text suffixes which will trigger a task transition into this status
     when mentioned in a commit message, after a valid prefix. For example,
     providing "as invalid" here will allow users to move tasks
     to this status by writing `Closes T123 as invalid`, even if another status
     is selected by the "Closes" prefix.
   - `keywords` //Optional list<string>.// Allows you to specify a list
     of keywords which can be used with `!status` commands in email to select
     this status.
   - `disabled` //Optional bool.// Marks this status as no longer in use so
     tasks can not be created or edited to have this status. Existing tasks with
     this status will not be affected, but you can batch edit them or let them
     die out on their own.
   - `claim` //Optional bool.// By default, closing an unassigned task claims
     it. You can set this to `false` to disable this behavior for a particular
     status.
   - `locked` //Optional string.// Lock tasks in this status. Specify "comments"
     to lock comments (users who can edit the task may override this lock).
     Specify "edits" to prevent anyone except the task owner from making edits.
   - `mfa` //Optional bool.// Require all edits to this task to be signed with
     multi-factor authentication.
 
 Statuses will appear in the UI in the order specified. Note the status marked
 `special` as `duplicate` is not settable directly and will not appear in UI
 elements, and that any status marked `silly` does not appear if Phabricator
 is configured with `phabricator.serious-business` set to true.
 
 Examining the default configuration and examples below will probably be helpful
 in understanding these options.
 
 EOTEXT
 ));
 
     $status_example = array(
       'open' => array(
         'name' => pht('Open'),
         'special' => 'default',
       ),
       'closed' => array(
         'name' => pht('Closed'),
         'special' => 'closed',
         'closed' => true,
       ),
       'duplicate' => array(
         'name' => pht('Duplicate'),
         'special' => 'duplicate',
         'closed' => true,
       ),
     );
 
     $json = new PhutilJSON();
     $status_example = $json->encodeFormatted($status_example);
 
     // This is intentionally blank for now, until we can move more Maniphest
     // logic to custom fields.
     $default_fields = array();
 
     foreach ($default_fields as $key => $enabled) {
       $default_fields[$key] = array(
         'disabled' => !$enabled,
       );
     }
 
     $custom_field_type = 'custom:PhabricatorCustomFieldConfigOptionType';
 
     $fields_example = array(
       'mycompany.estimated-hours' => array(
         'name' => pht('Estimated Hours'),
         'type' => 'int',
         'caption' => pht('Estimated number of hours this will take.'),
       ),
     );
     $fields_json = id(new PhutilJSON())->encodeFormatted($fields_example);
 
     $points_type = 'maniphest.points';
 
     $points_example_1 = array(
       'enabled' => true,
       'label' => pht('Story Points'),
       'action' => pht('Change Story Points'),
     );
     $points_json_1 = id(new PhutilJSON())->encodeFormatted($points_example_1);
 
     $points_example_2 = array(
       'enabled' => true,
       'label' => pht('Estimated Hours'),
       'action' => pht('Change Estimate'),
     );
     $points_json_2 = id(new PhutilJSON())->encodeFormatted($points_example_2);
 
     $points_description = $this->deformat(pht(<<<EOTEXT
 Activates a points field on tasks. You can use points for estimation or
 planning. If configured, points will appear on workboards.
 
 To activate points, set this value to a map with these keys:
 
   - `enabled` //Optional bool.// Use `true` to enable points, or
     `false` to disable them.
   - `label` //Optional string.// Label for points, like "Story Points" or
     "Estimated Hours". If omitted, points will be called "Points".
   - `action` //Optional string.// Label for the action which changes points
     in Maniphest, like "Change Estimate". If omitted, the action will
     be called "Change Points".
 
 See the example below for a starting point.
 EOTEXT
 ));
 
     $subtype_type = 'maniphest.subtypes';
     $subtype_default_key = PhabricatorEditEngineSubtype::SUBTYPE_DEFAULT;
     $subtype_example = array(
       array(
         'key' => $subtype_default_key,
         'name' => pht('Task'),
       ),
       array(
         'key' => 'bug',
         'name' => pht('Bug'),
       ),
       array(
         'key' => 'feature',
         'name' => pht('Feature Request'),
       ),
     );
     $subtype_example = id(new PhutilJSON())->encodeAsList($subtype_example);
 
     $subtype_default = array(
       array(
         'key' => $subtype_default_key,
         'name' => pht('Task'),
       ),
     );
 
     $subtype_description = $this->deformat(pht(<<<EOTEXT
 Allows you to define task subtypes. Subtypes let you hide fields you don't
 need to simplify the workflows for editing tasks.
 
 To define subtypes, provide a list of subtypes. Each subtype should be a
 dictionary with these keys:
 
   - `key` //Required string.// Internal identifier for the subtype, like
     "task", "feature", or "bug".
   - `name` //Required string.// Human-readable name for this subtype, like
     "Task", "Feature Request" or "Bug Report".
   - `tag` //Optional string.// Tag text for this subtype.
   - `color` //Optional string.// Display color for this subtype.
   - `icon` //Optional string.// Icon for the subtype.
   - `children` //Optional map.// Configure options shown to the user when
      they "Create Subtask". See below.
   - `fields` //Optional map.// Configure field behaviors. See below.
+  - `mutations` //Optional list.// Configure which subtypes this subtype
+    can easily be converted to by using the "Change Subtype" action. See below.
 
 Each subtype must have a unique key, and you must define a subtype with
 the key "%s", which is used as a default subtype.
 
 The tag text (`tag`) is used to set the text shown in the subtype tag on list
 views and workboards. If you do not configure it, the default subtype will have
 no subtype tag and other subtypes will use their name as tag text.
 
 The `children` key allows you to configure which options are presented to the
 user when they "Create Subtask" from a task of this subtype. You can specify
 these keys:
 
   - `subtypes`: //Optional list<string>.// Show users creation forms for these
     task subtypes.
   - `forms`: //Optional list<string|int>.// Show users these specific forms,
     in order.
 
 If you don't specify either constraint, users will be shown creation forms
 for the same subtype.
 
 For example, if you have a "quest" subtype and do not configure `children`,
 users who click "Create Subtask" will be presented with all create forms for
 "quest" tasks.
 
 If you want to present them with forms for a different task subtype or set of
 subtypes instead, use `subtypes`:
 
 ```
   {
     ...
     "children": {
       "subtypes": ["objective", "boss", "reward"]
     }
     ...
   }
 ```
 
 If you want to present them with specific forms, use `forms` and specify form
 IDs:
 
 ```
   {
     ...
     "children": {
       "forms": [12, 16]
     }
     ...
   }
 ```
 
 When specifying forms by ID explicitly, the order you specify the forms in will
 be used when presenting options to the user.
 
 If only one option would be presented, the user will be taken directly to the
 appropriate form instead of being prompted to choose a form.
 
 The `fields` key can configure the behavior of custom fields on specific
 task subtypes. For example:
 
 ```
-{
-  ...
-  "fields": {
-    "custom.some-field": {
-      "disabled": true
+  {
+    ...
+    "fields": {
+      "custom.some-field": {
+        "disabled": true
+      }
     }
+    ...
   }
-  ...
-}
 ```
 
 Each field supports these options:
 
   - `disabled` //Optional bool.// Allows you to disable fields on certain
     subtypes.
   - `name` //Optional string.// Custom name of this field for the subtype.
 
+
+The `mutations` key allows you to control the behavior of the "Change Subtype"
+action above the comment area. By default, this action allows users to change
+the task subtype into any other subtype.
+
+If you'd prefer to make it more difficult to change subtypes or offer only a
+subset of subtypes, you can specify the list of subtypes that "Change Subtypes"
+offers. For example, if you have several similar subtypes and want to allow
+tasks to be converted between them but not easily converted to other types,
+you can make the "Change Subtypes" control show only these options like this:
+
+```
+  {
+    ...
+    "mutations": ["bug", "issue", "defect"]
+    ...
+  }
+```
+
+If you specify an empty list, the "Change Subtypes" action will be completely
+hidden.
+
+This mutation list is advisory and only configures the UI. Tasks may still be
+converted across subtypes freely by using the Bulk Editor or API.
+
 EOTEXT
       ,
       $subtype_default_key));
 
     $priorities_description = $this->deformat(pht(<<<EOTEXT
 Allows you to edit or override the default priorities available in Maniphest,
 like "High", "Normal" and "Low". The configuration should contain a map of
 numeric priority values (where larger numbers correspond to higher priorities)
 to priority specifications (see defaults below for examples).
 
 The keys you can define for a priority are:
 
   - `name` //Required string.// Name of the priority.
   - `keywords` //Required list<string>.// List of unique keywords which identify
     this priority, like "high" or "low". Each priority must have at least one
     keyword and two priorities may not share the same keyword.
   - `short` //Optional string.// Alternate shorter name, used in UIs where
     there is less space available.
   - `color` //Optional string.// Color for this priority, like "red" or
     "blue".
   - `disabled` //Optional bool.// Set to true to prevent users from choosing
     this priority when creating or editing tasks. Existing tasks will not be
     affected, and can be batch edited to a different priority or left to
     eventually die out.
 
 You can choose the default priority for newly created tasks with
 "maniphest.default-priority".
 EOTEXT
       ));
 
     $fields_description = $this->deformat(pht(<<<EOTEXT
 List of custom fields for Maniphest tasks.
 
 For details on adding custom fields to Maniphest, see [[ %s | %s ]] in the
 documentation.
 EOTEXT
       ,
       PhabricatorEnv::getDoclink('Configuring Custom Fields'),
       pht('Configuring Custom Fields')));
 
     return array(
       $this->newOption('maniphest.custom-field-definitions', 'wild', array())
         ->setSummary(pht('Custom Maniphest fields.'))
         ->setDescription($fields_description)
         ->addExample($fields_json, pht('Valid setting')),
       $this->newOption('maniphest.fields', $custom_field_type, $default_fields)
         ->setCustomData(id(new ManiphestTask())->getCustomFieldBaseClass())
         ->setDescription(pht('Select and reorder task fields.')),
       $this->newOption(
         'maniphest.priorities',
         $priority_type,
         $priority_defaults)
         ->setSummary(pht('Configure Maniphest priority names.'))
         ->setDescription($priorities_description),
       $this->newOption('maniphest.statuses', $status_type, $status_defaults)
         ->setSummary(pht('Configure Maniphest task statuses.'))
         ->setDescription($status_description)
         ->addExample($status_example, pht('Minimal Valid Config')),
       $this->newOption('maniphest.default-priority', 'int', 90)
         ->setSummary(pht('Default task priority for create flows.'))
         ->setDescription(
           pht(
             'Choose a default priority for newly created tasks. You can '.
             'review and adjust available priorities by using the '.
             '%s configuration option. The default value (`90`) '.
             'corresponds to the default "Needs Triage" priority.',
             'maniphest.priorities')),
       $this->newOption('maniphest.points', $points_type, array())
         ->setSummary(pht('Configure point values for tasks.'))
         ->setDescription($points_description)
         ->addExample($points_json_1, pht('Points Config'))
         ->addExample($points_json_2, pht('Hours Config')),
       $this->newOption('maniphest.subtypes', $subtype_type, $subtype_default)
         ->setSummary(pht('Define task subtypes.'))
         ->setDescription($subtype_description)
         ->addExample($subtype_example, pht('Simple Subtypes')),
     );
   }
 
 }
diff --git a/src/applications/transactions/editengine/PhabricatorEditEngineSubtype.php b/src/applications/transactions/editengine/PhabricatorEditEngineSubtype.php
index 6e1d1de115..f471fcd92f 100644
--- a/src/applications/transactions/editengine/PhabricatorEditEngineSubtype.php
+++ b/src/applications/transactions/editengine/PhabricatorEditEngineSubtype.php
@@ -1,281 +1,316 @@
 <?php
 
 
 final class PhabricatorEditEngineSubtype
   extends Phobject {
 
   const SUBTYPE_DEFAULT = 'default';
 
   private $key;
   private $name;
   private $icon;
   private $tagText;
   private $color;
   private $childSubtypes = array();
   private $childIdentifiers = array();
   private $fieldConfiguration = array();
+  private $mutations;
 
   public function setKey($key) {
     $this->key = $key;
     return $this;
   }
 
   public function getKey() {
     return $this->key;
   }
 
   public function setName($name) {
     $this->name = $name;
     return $this;
   }
 
   public function getName() {
     return $this->name;
   }
 
   public function setIcon($icon) {
     $this->icon = $icon;
     return $this;
   }
 
   public function getIcon() {
     return $this->icon;
   }
 
   public function setTagText($text) {
     $this->tagText = $text;
     return $this;
   }
 
   public function getTagText() {
     return $this->tagText;
   }
 
   public function setColor($color) {
     $this->color = $color;
     return $this;
   }
 
   public function getColor() {
     return $this->color;
   }
 
   public function setChildSubtypes(array $child_subtypes) {
     $this->childSubtypes = $child_subtypes;
     return $this;
   }
 
   public function getChildSubtypes() {
     return $this->childSubtypes;
   }
 
   public function setChildFormIdentifiers(array $child_identifiers) {
     $this->childIdentifiers = $child_identifiers;
     return $this;
   }
 
   public function getChildFormIdentifiers() {
     return $this->childIdentifiers;
   }
 
+  public function setMutations($mutations) {
+    $this->mutations = $mutations;
+    return $this;
+  }
+
+  public function getMutations() {
+    return $this->mutations;
+  }
+
   public function hasTagView() {
     return (bool)strlen($this->getTagText());
   }
 
   public function newTagView() {
     $view = id(new PHUITagView())
       ->setType(PHUITagView::TYPE_OUTLINE)
       ->setName($this->getTagText());
 
     $color = $this->getColor();
     if ($color) {
       $view->setColor($color);
     }
 
     return $view;
   }
 
   public function setSubtypeFieldConfiguration(
     $subtype_key,
     array $configuration) {
     $this->fieldConfiguration[$subtype_key] = $configuration;
     return $this;
   }
 
   public function getSubtypeFieldConfiguration($subtype_key) {
     return idx($this->fieldConfiguration, $subtype_key);
   }
 
   public static function validateSubtypeKey($subtype) {
     if (strlen($subtype) > 64) {
       throw new Exception(
         pht(
           'Subtype "%s" is not valid: subtype keys must be no longer than '.
           '64 bytes.',
           $subtype));
     }
 
     if (strlen($subtype) < 3) {
       throw new Exception(
         pht(
           'Subtype "%s" is not valid: subtype keys must have a minimum '.
           'length of 3 bytes.',
           $subtype));
     }
 
     if (!preg_match('/^[a-z]+\z/', $subtype)) {
       throw new Exception(
         pht(
           'Subtype "%s" is not valid: subtype keys may only contain '.
           'lowercase latin letters ("a" through "z").',
           $subtype));
     }
   }
 
   public static function validateConfiguration($config) {
     if (!is_array($config)) {
       throw new Exception(
         pht(
           'Subtype configuration is invalid: it must be a list of subtype '.
           'specifications.'));
     }
 
     $map = array();
     foreach ($config as $value) {
       PhutilTypeSpec::checkMap(
         $value,
         array(
           'key' => 'string',
           'name' => 'string',
           'tag' => 'optional string',
           'color' => 'optional string',
           'icon' => 'optional string',
           'children' => 'optional map<string, wild>',
           'fields' => 'optional map<string, wild>',
+          'mutations' => 'optional list<string>',
         ));
 
       $key = $value['key'];
       self::validateSubtypeKey($key);
 
       if (isset($map[$key])) {
         throw new Exception(
           pht(
             'Subtype configuration is invalid: two subtypes use the same '.
             'key ("%s"). Each subtype must have a unique key.',
             $key));
       }
 
       $map[$key] = true;
 
       $name = $value['name'];
       if (!strlen($name)) {
         throw new Exception(
           pht(
             'Subtype configuration is invalid: subtype with key "%s" has '.
             'no name. Subtypes must have a name.',
             $key));
       }
 
       $children = idx($value, 'children');
       if ($children) {
         PhutilTypeSpec::checkMap(
           $children,
           array(
             'subtypes' => 'optional list<string>',
             'forms' => 'optional list<string|int>',
           ));
 
         $child_subtypes = idx($children, 'subtypes');
         $child_forms = idx($children, 'forms');
 
         if ($child_subtypes && $child_forms) {
           throw new Exception(
             pht(
               'Subtype configuration is invalid: subtype with key "%s" '.
               'specifies both child subtypes and child forms. Specify one '.
               'or the other, but not both.'));
         }
       }
 
       $fields = idx($value, 'fields');
       if ($fields) {
         foreach ($fields as $field_key => $configuration) {
           PhutilTypeSpec::checkMap(
             $configuration,
             array(
               'disabled' => 'optional bool',
               'name' => 'optional string',
             ));
         }
       }
     }
 
     if (!isset($map[self::SUBTYPE_DEFAULT])) {
       throw new Exception(
         pht(
           'Subtype configuration is invalid: there is no subtype defined '.
           'with key "%s". This subtype is required and must be defined.',
           self::SUBTYPE_DEFAULT));
     }
+
+    foreach ($config as $value) {
+      $key = idx($value, 'key');
+
+      $mutations = idx($value, 'mutations');
+      if (!$mutations) {
+        continue;
+      }
+
+      foreach ($mutations as $mutation) {
+        if (!isset($map[$mutation])) {
+          throw new Exception(
+            pht(
+              'Subtype configuration is invalid: subtype with key "%s" '.
+              'specifies that it can mutate into subtype "%s", but that is '.
+              'not a valid subtype.',
+              $key,
+              $mutation));
+        }
+      }
+    }
+
   }
 
   public static function newSubtypeMap(array $config) {
     $map = array();
 
     foreach ($config as $entry) {
       $key = $entry['key'];
       $name = $entry['name'];
 
       $tag_text = idx($entry, 'tag');
       if ($tag_text === null) {
         if ($key != self::SUBTYPE_DEFAULT) {
           $tag_text = phutil_utf8_strtoupper($name);
         }
       }
 
       $color = idx($entry, 'color', 'blue');
       $icon = idx($entry, 'icon', 'fa-drivers-license-o');
 
       $subtype = id(new self())
         ->setKey($key)
         ->setName($name)
         ->setTagText($tag_text)
         ->setIcon($icon);
 
       if ($color) {
         $subtype->setColor($color);
       }
 
       $children = idx($entry, 'children', array());
       $child_subtypes = idx($children, 'subtypes');
       $child_forms = idx($children, 'forms');
 
       if ($child_subtypes) {
         $subtype->setChildSubtypes($child_subtypes);
       }
 
       if ($child_forms) {
         $subtype->setChildFormIdentifiers($child_forms);
       }
 
       $field_configurations = idx($entry, 'fields');
       if ($field_configurations) {
         foreach ($field_configurations as $field_key => $field_configuration) {
           $subtype->setSubtypeFieldConfiguration(
             $field_key,
             $field_configuration);
         }
       }
 
+      $subtype->setMutations(idx($entry, 'mutations'));
+
       $map[$key] = $subtype;
     }
 
     return new PhabricatorEditEngineSubtypeMap($map);
   }
 
   public function newIconView() {
     return id(new PHUIIconView())
       ->setIcon($this->getIcon(), $this->getColor());
   }
 
 }
diff --git a/src/applications/transactions/editengine/PhabricatorEditEngineSubtypeMap.php b/src/applications/transactions/editengine/PhabricatorEditEngineSubtypeMap.php
index dc1ee2842a..edf2d2045a 100644
--- a/src/applications/transactions/editengine/PhabricatorEditEngineSubtypeMap.php
+++ b/src/applications/transactions/editengine/PhabricatorEditEngineSubtypeMap.php
@@ -1,97 +1,135 @@
 <?php
 
 
 final class PhabricatorEditEngineSubtypeMap
   extends Phobject {
 
   private $subtypes;
   private $datasource;
 
   public function __construct(array $subtypes) {
     assert_instances_of($subtypes, 'PhabricatorEditEngineSubtype');
 
     $this->subtypes = $subtypes;
   }
 
   public function getDisplayMap() {
     return mpull($this->subtypes, 'getName');
   }
 
   public function getCount() {
     return count($this->subtypes);
   }
 
   public function isValidSubtype($subtype_key) {
     return isset($this->subtypes[$subtype_key]);
   }
 
   public function getSubtypes() {
     return $this->subtypes;
   }
 
   public function getSubtype($subtype_key) {
     if (!$this->isValidSubtype($subtype_key)) {
       throw new Exception(
         pht(
           'Subtype key "%s" does not identify a valid subtype.',
           $subtype_key));
     }
 
     return $this->subtypes[$subtype_key];
   }
 
   public function setDatasource(PhabricatorTypeaheadDatasource $datasource) {
     $this->datasource = $datasource;
     return $this;
   }
 
   public function newDatasource() {
     if (!$this->datasource) {
       throw new PhutilInvalidStateException('setDatasource');
     }
 
     return clone($this->datasource);
   }
 
+  public function getMutationMap($source_key) {
+    return mpull($this->getMutations($source_key), 'getName');
+  }
+
+  public function getMutations($source_key) {
+    $mutations = $this->subtypes;
+
+    $subtype = idx($this->subtypes, $source_key);
+    if ($subtype) {
+      $map = $subtype->getMutations();
+      if ($map !== null) {
+        $map = array_fuse($map);
+        foreach ($mutations as $key => $mutation) {
+          if ($key === $source_key) {
+            // This is the current subtype, so we always want to show it.
+            continue;
+          }
+
+          if (isset($map[$key])) {
+            // This is an allowed mutation, so keep it.
+            continue;
+          }
+
+          // Discard other subtypes as mutation options.
+          unset($mutations[$key]);
+        }
+      }
+    }
+
+    // If the only available mutation is the current subtype, treat this like
+    // no mutations are available.
+    if (array_keys($mutations) === array($source_key)) {
+      $mutations = array();
+    }
+
+    return $mutations;
+  }
+
   public function getCreateFormsForSubtype(
     PhabricatorEditEngine $edit_engine,
     PhabricatorEditEngineSubtypeInterface $object) {
 
     $subtype_key = $object->getEditEngineSubtype();
     $subtype = $this->getSubtype($subtype_key);
 
     $select_identifiers = $subtype->getChildFormIdentifiers();
     $select_subtypes = $subtype->getChildSubtypes();
 
     $query = $edit_engine->newConfigurationQuery()
       ->withIsDisabled(false);
 
     if ($select_identifiers) {
       $query->withIdentifiers($select_identifiers);
     } else {
       // If we're selecting by subtype rather than selecting specific forms,
       // only select create forms.
       $query->withIsDefault(true);
 
       if ($select_subtypes) {
         $query->withSubtypes($select_subtypes);
       } else {
         $query->withSubtypes(array($subtype_key));
       }
     }
 
     $forms = $query->execute();
     $forms = mpull($forms, null, 'getIdentifier');
 
     // If we're selecting by ID, respect the order specified in the
     // constraint. Otherwise, use the create form sort order.
     if ($select_identifiers) {
       $forms = array_select_keys($forms, $select_identifiers) + $forms;
     } else {
       $forms = msort($forms, 'getCreateSortKey');
     }
 
     return $forms;
   }
 
 }
diff --git a/src/applications/transactions/engineextension/PhabricatorSubtypeEditEngineExtension.php b/src/applications/transactions/engineextension/PhabricatorSubtypeEditEngineExtension.php
index d0b6d017f3..e73d476d74 100644
--- a/src/applications/transactions/engineextension/PhabricatorSubtypeEditEngineExtension.php
+++ b/src/applications/transactions/engineextension/PhabricatorSubtypeEditEngineExtension.php
@@ -1,60 +1,68 @@
 <?php
 
 final class PhabricatorSubtypeEditEngineExtension
   extends PhabricatorEditEngineExtension {
 
   const EXTENSIONKEY = 'editengine.subtype';
   const EDITKEY = 'subtype';
 
   public function getExtensionPriority() {
     return 8000;
   }
 
   public function isExtensionEnabled() {
     return true;
   }
 
   public function getExtensionName() {
     return pht('Subtypes');
   }
 
   public function supportsObject(
     PhabricatorEditEngine $engine,
     PhabricatorApplicationTransactionInterface $object) {
     return $engine->supportsSubtypes();
   }
 
   public function buildCustomEditFields(
     PhabricatorEditEngine $engine,
     PhabricatorApplicationTransactionInterface $object) {
 
     $subtype_type = PhabricatorTransactions::TYPE_SUBTYPE;
+    $subtype_value = $object->getEditEngineSubtype();
 
     $map = $object->newEditEngineSubtypeMap();
-    $options = $map->getDisplayMap();
+
+    if ($object->getID()) {
+      $options = $map->getMutationMap($subtype_value);
+    } else {
+      // NOTE: This is a crude proxy for "are we in the bulk edit workflow".
+      // We want to allow any mutation.
+      $options = $map->getDisplayMap();
+    }
 
     $subtype_field = id(new PhabricatorSelectEditField())
       ->setKey(self::EDITKEY)
       ->setLabel(pht('Subtype'))
       ->setIsFormField(false)
       ->setTransactionType($subtype_type)
       ->setConduitDescription(pht('Change the object subtype.'))
       ->setConduitTypeDescription(pht('New object subtype key.'))
-      ->setValue($object->getEditEngineSubtype())
+      ->setValue($subtype_value)
       ->setOptions($options);
 
-    // If subtypes are configured, enable changing them from the bulk editor
-    // and comment action stack.
-    if ($map->getCount() > 1) {
+    // If subtypes are configured, enable changing them from the bulk editor.
+    // Bulk editor
+    if ($options) {
       $subtype_field
         ->setBulkEditLabel(pht('Change subtype to'))
         ->setCommentActionLabel(pht('Change Subtype'))
         ->setCommentActionOrder(3000);
     }
 
     return array(
       $subtype_field,
     );
   }
 
 }